Photo-detectors are widely used for sensing light radiation. Typically, the photo-detector should be sensitive in some given spectral region (range of optical wavelengths). In some cases, the responsivity of the photo-detector should be constant or at least well defined within some wavelength range. It can also be important to have zero response in some other wavelength range; an example is solar-blind detectors, being sensitive only to short-wavelength ultraviolet light but not to sun light.
The photo-detector should be suitable for some range of optical powers. The maximum detected power can be limited e.g. by damage issues or by a nonlinear response, whereas the minimum power is normally determined by noise. The magnitude of the dynamic range (typically specified as the ratio of maximum and minimum detectable power, e.g. in decibels) is often most important. Some photo-detectors (e.g. photodiodes) can exhibit high linearity over a dynamic range of more than 70 dB.
In some cases, not only a high responsivity, but also a high quantum efficiency is important, as otherwise additional quantum noise is introduced. This applies e.g. to the detection of squeezed states of light, and also affects the photon detection probability of single-photon detectors.
The active area of a photo-detector can be important e.g. when working with strongly divergent beams from laser diodes. For light sources with very high and/or non-constant beam divergence, it is hardly possible to get all the light onto the active area. An integrating sphere may then be used (with appropriate calibration) for measuring the total power.
The detection bandwidth may begin at 0 Hz or some finite frequency, and ends at some maximum frequency which may be limited by internal processes (e.g. the speed of electric carriers in a semiconductor material) or by the involved electronics (e.g. introducing some RC time constants). Some resonant photo-detectors operate only in a narrow frequency range, and can be suitable e.g. for lock-in detection.